Ferritic stainless steels have excellent resistance to some strongly oxidizing substances but are quite soft and subject to rapid erosion or abrasion. Martensitic stainless steels may be produced in a hard state but have very limited corrosion resistance and have not performed well in most abrasive slurries. Austenitic stainless steels and irons resist the chemical attack of a wide spectrum of corrosive substances, however, they are relatively soft metals and are rapidly abraded or eroded in fluids which contain abrasive particles or even vapor bubbles.
While certain highly modified stainless alloys have been developed to handle various strengths of sulfuric acid, they are quite soft and abrade rapidly in erosive slurries containing hard particles and/or air bubbles. A minimum of about 350 to 400 Brinell Hardness number (BHN) has been found to be required for good abrasion resistance to most slurries.
White cast irons, particularly those of largely martensitic matrices, have been employed for decades in applications involving dry or wet abrasive substances. Many fluids or slurries, such as highly acidic slurries, are both abrasive and corrosive to metal parts designed to handle them and low to medium alloy white cast irons are rapidly attacked by most corrosive slurries.
Under certain conditions of combined corrosion and abrasion, high carbon high chromium cast irons have been widely employed since about 1930. For example, high chromium cast irons with chromium contents over 20% give excellent resistance to nitric acid up to about 66% acid strength at room temperature and up to about 37% acid strengths to the boiling temperature. They are also useful in phosphoric acid concentrations up to about 45%, in sodium hydroxide up to 15%, ammonium nitrate or sulphate up to 50% and several other types of corrosive substances. They do not have useful resistance to sulfuric acid of any strength up to 60%. They could be employed in cold concentrated sulfuric acid but offer no advantage over ordinary plain gray cast iron in this service. High chromium irons typically contain about 2.5% carbon, 28% chromium and no nickel. In the as cast condition castings of these irons typically range from about 450 to 580 Brinell Hardness numbers (BHN). These irons may be slightly softened for improved machinability by annealing heat treatments and when machining is finished, may be given different heat treatments for improved abrasion resistance and hardnesses of up to about 550 to 700 BHN.
Heyer, et al. U.S. Pat. No. 4,080,198, discloses an iron which nominally contains 1.6% C, 28% Cr, 2% Mo, 2% Ni, up to 1% Cu and the balance substantially iron plus small amounts of impurities or tramp elements. This alloy is claimed to have corrosion-erosion resistance superior to the nickel-free, higher-carbon alloys described above in acidic or saline slurries. Khandros, et al., U.S. Pat. No. 4,536,232, discloses an iron of the same chemical composition described by Heyer et al., but is said to have greatly improved corrosion and erosion resistance over the '198 alloy in 20% aluminum slurries of 0.75 pH due to the addition of 2.5% sulfuric acid. The improvement is said to have been obtained by a special heat treatment, i.e., double tempering at about 1400.degree. F. This alloy, whether heat treated according to the '198 patent or according to the '232, or left in the as-cast state, is always composed of large amounts of carbides precipitated in and around grains of mixed matrix structure, which are principally magnetic and of body center cubic crystal cell formation. Machining is quite difficult regardless of heat treatment.
The microscopic carbide particles of high carbon alloys have hardnesses of about 1700 to 1800, as measured by the pointed diamond pyramid method. However, the Brinell Hardness test employs a 10 mm diameter hard ball which takes into account the effect of the hard carbides and the soft matrices of these alloys in which the carbides are embedded. While the carbides provide abrasion resistance in high carbon alloys, it is well established that microscopic soft matrix areas between the hard carbide particles must have good resistance to whatever corrosive substances might be present in the slurries, if long alloy life is to be realized.
Niu Hong-jun, et al., in "Heat-Resisting Materials, Proceedings of the First International Conference," Fontana, Wisc., USA/Sep. 23-26, 1991, pp. 269-274, discloses an alloy having a composition by weight percentages, of 1.5% C, 18.2% Cr, 6.9% Ni, 2.5% Mo, 2.6% Si, 0.99% Mn, 0.16% P, 0.035% S and the balance substantially iron. The structure of this alloy as cast is composed of carbides and austenite (face center cubic crystal structure matrix). The alloy was developed for corrosion resistance, wear resistance and hot hardness as an automotive engine valve seat insert material.
Certain cobalt base alloys have demonstrated good resistance to both the abrasion and corrosion encountered in handling some corrosive slurries. However, cobalt is quite scarce and far too expensive for the large tonnages of pumps and other parts required in the handling of corrosive industrial slurries. Aside from the economically impractical cobalt base alloys, stainless steels and similar alloys have good corrosion resistance but very poor abrasion resistance, while non-austenitic high alloy cast irons have excellent abrasion resistance but very limited corrosion resistance. The high-carbon austenitic steel of Niu Hong-jun, et al., has good abrasion resistance but very limited corrosion resistance. Thus there has remains a great need for alloys of reasonable cost that are able to resist both the abrasion and corrosion of chemically aggressive slurries and similar fluids and which do not require that articles cast from such alloys undergo extensive and expensive heat treatment.